The present invention relates to medical ultrasound diagnostic equipment. In particular it relates to a Doppler blood velocity meter.
In the measurement of blood velocity using pulsed ultrasound Doppler equipment, there is a recognized problem in measuring the velocity of blood in deep lying vessels. The problem results from the fact that the pulse repetition frequency is determined, in part, by the depth within the body of the blood whose velocity is being measured. The pulse repetition frequency is typically selected such that a pulse can be transmitted from the transducer and reflected from blood flowing within the vessel with the return pulse being received prior to the transmission of the next succeeding pulse.
As used herein, the term "PRF" means the pulse repetition frequency, and the term "sample volume" means the region of interest of blood flow velocity. These terms are well known and understood in the art. As will be recognized by those skilled in the art, the maximum PRF which can be used without introducing depth ambiguities is equal to the speed of sound in the medium divided by twice the depth of the sample volume. A phenomenon which has been observed and which is well recognized, called "aliasing", occurs when blood flow exceeds a maximum velocity for a given ultrasound transmitted frequency. This results from the fact that the Doppler shift frequency is equal to twice the ultrasound transmitted frequency times the velocity of the moving blood divided by the velocity of sound in the body times the cosine of the angle between the moving blood and the insonifying sound wave. When the Doppler frequency is more than one half the PRF, the phenomenon of aliasing, wherein the blood flow appears to have a different velocity or direction then it actually has, will be observed. The aliasing phenomenon occurs when the maximum blood flow velocity is greater than or equal to the square of the speed of sound in the human body divided by product of eight times the ultrasound transmitted frequency times the sample volume body times the cosine of the angle between the moving blood and the insonifying sound wave. In other words, a maximum velocity of blood flow which can be measured without exhibiting aliasing is inversely proportional to the sample volume depth in the body for a given ultrasound transmitted frequency.
While one approach to increasing the maximum velocity which can be measured without aliasing is to reduce the frequency of the ultrasound transmitted energy, if the frequency decreases below about 2 MHz, the scattering phenomenon, which is required for observing the returned Doppler signals, is degraded. In addition, a reduction in the ultrasound transmitted frequency reduces also the resolution of the sample volume. Accordingly, while the approach of decreasing the frequency of the ultrasound transmitted frequency can be helpful to about 2 MHz, thereafter it has been found not to be a desirable approach to use for eliminating the aliasing effect. Accordingly, a new approach to providing an unaliased signal would be desirable.